Method and system for monitoring and identifying moisture intrusion in soil such as is contained in landfills housing radioactive and/or hazardous waste. The invention utilizes the principle that moist or wet soil has a higher thermal conductance than dry soil. The invention employs optical time delay reflectometry in connection with a distributed temperature sensing system together with heating means in order to identify discrete areas within a volume of soil wherein temperature is lower. According to the invention an optical element and, optionally, a heating element may be included in a cable or other similar structure and arranged in a serpentine fashion within a volume of soil to achieve efficient temperature detection across a large area or three dimensional volume of soil. Remediation, moisture countermeasures, or other responsive action may then be coordinated based on the assumption that cooler regions within a soil volume may signal moisture intrusion where those regions are located.
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13. A method of soil characterization comprising the steps of:
identifying at least one region within a volume of said soil, which has a lower temperature than at least one other region within said volume of said soil; and defining said at least one region having a lower temperature as having potentially greater concentration of moisture as compared with other regions.
1. A soil moisture sensing system comprising:
a linear element positioned in a volume of soil, said linear element having a thickness and a length, said linear element comprising a distributed temperature sensing element coextensive with at least a portion of the length of said linear element, said distributed temperature sensing element being adapted to register changes in temperature of said soil including changes apparent following alteration of thermal conductance of said soil; and a heater which, when actuated, causes an increase in temperature of at least a portion of said soil, which increase is detectable by at least part of said distributed temperature sensing element.
8. A method for detecting moisture intrusion in soil comprising the steps of:
positioning in soil a linear element having a thickness and a length, said length being of a dimension greater than that of said thickness, said linear element comprising a distributed temperature sensing element coextensive with at least a portion of the length of said linear element, heating at least a portion of said soil, and measuring temperatures within said volume of soil, using said distributed temperature sensing element, to determine if temperature differences are apparent indicating presence of regions within said volume possibly having differing thermal conductance characteristics which respond differently to said heating.
2. The soil moisture sensing system of
3. The soil moisture sensing system of
4. The soil moisture sensing system of
5. The soil moisture sensing system of
6. The soil moisture sensing system of
7. The soil moisture sensing system of
9. The method of
transmitting at least one pulse of radiation through an optical fiber having a length, said optical fiber being coextensive with at least a portion of said linear element, whereby Raman scattered radiation is generated along the length of said optical fiber, said Raman scattered radiation comprising at least two separate wavelengths exhibiting different temperature dependencies, and comparing signals detected from the at least two separate wavelengths to determine temperature at a plurality of locations from which said Raman scattered radiation is generated along the length of said optical fiber.
10. The method of
12. The method of
14. The method according to
15. The method of
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This invention was made with support from the United States Government under Contract DE-AC04-96AL85000 awarded by the U.S. Department of Energy. The Government has certain rights in this invention.
1. Field of the Invention
This invention pertains generally to moisture detection in soil, and more specifically to a method and apparatus using principles of thermal conductance to detect intrusion of moisture into soil and landfills containing toxic and/or radioactive waste.
2. Description of the Related Art
Thousands of landfills exist across the U.S. which are operated by both government and private entities. Many such landfills contain chemical waste, low-level radioactive waste and/or mixed waste. Moisture intrusion into the soil comprising such landfills represents a problem in that wastes can be destabilized or even mobilized resulting in inadequate containment. Moisture, from precipitation (rain and snow, for example), or from overland runoff can mobilize waste resulting in contamination of ground water sources thereby creating a health hazard. Therefore, effective and economical monitoring of the presence and movement of moisture in landfills is often critical to environmental safety and remediation programs.
Various containment methods including using barrier covers, liners and in-situ grouting have been proposed and used in many government and private remediation sites. Additionally, though, long term (10's of years) and short term (0 to 5 years) monitoring is required to address concerns of stakeholders as well as to satisfy regulatory requirements. Fundamental to many containment approaches is the need for a clear understanding of how fluids move through, accumulate in, and leave soils (or subsurface wastes and containment structures).
A need remains, therefore, for a reliable sensor system that can detect in situ the intrusion of moisture in soil wherein hazardous and/or radioactive waste is stored. The present invention aids in this understanding by providing a method and apparatus for monitoring fluid flow in situ.
It is an object of the present invention to provide a sensing system and method that detects moisture intrusion in soil using a simple apparatus operating in situ in soil containing wastes of concern.
It is another object of the present invention to provide a sensing system and method that infers the presence of moisture based on temperature differences detectable using a distributed temperature sensing element.
It is yet another object of the invention to rely on different thermal conductances of dry soil, slightly moist soil and saturated soil to detect presence and location of moisture in soil.
It is yet another object of the invention to utilize application of heat to soil in order to isolate where regions having different thermal conductances are located in a volume of soil being investigated (sometimes referred to as the "target area" in this disclosure).
Yet another object is to provide a soil moisture sensing system that includes a linear element (such as a tube, cable or conduit) positioned in a volume of soil. Another related object is the provision of a distributed temperature sensing element associated with the linear element that permits identification of the regions having different thermal conductances mentioned above, especially when a heater is included, which, when actuated, heats the soil.
Yet another object of the invention is to provide a method for detecting moisture intrusion in soil which includes positioning in soil a linear element that comprises a distributed temperature sensing element, and further includes heating at least a portion of the soil, and measuring temperatures in the soil. Using the invention in this way, it is possible to ascertain whether regions are present in the soil suggesting different thermal conductances and, therefore, possibly the presence of moisture.
These and other objects are fulfilled and satisfied by the claimed invention which utilizes a linear element such as a cable, tube or conduit positioned in a serpentine fashion within a target area of soil. Optimal placement of the linear element assumes that it passes through a significant two-dimensional area and, preferably, also a significant three-dimensional volume within the target portion of the soil. The invention also includes using an optical fiber, that forms a component of a distributed temperature sensing system, and a heating element. The optical fiber serves as an optical conduit through which pulses of light pass, generating Raman scattered radiation within the target area. The scattered radiation is detected and analyzed using principles of optical time delay reflectometry (OTDR) to permit accurate and distributed temperature detection capability along the length of the fiber optic. The heating element which, in a primary embodiment disclosed here, is integrated with the cable or other linear element, is used to heat soil. Principles of the invention, though, would also be served using by an external heating apparatus. The heating causes regions of differing thermal conductance within the soil to become apparent and detectable by the distributed temperature detection system. According to the invention, those areas having relatively higher thermal conductance (possibly reflecting the presence of moisture) can thus be identified and located.
Additional advantages and novel features will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The accompanying drawings, which are incorporated into and form part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
Reliable and inexpensive detection of moisture intrusion, for example, in a landfill, is obtained using the present invention which involves heating soil slightly, and then monitoring temperatures at various points within the soil to identify regions exhibiting a relative decrease in temperature where moisture has penetrated the soil. This drop will often be quite dramatic because the thermal conductance of saturated, or near saturated, sand, for example, is so much greater than that of dry sand, where dead air spaces exist between the grains.
High resolution detection of differences in thermal conductance is possible through the present invention by its employment of OTDR principles, known to those skilled in the art of distributed temperature measurement. Novelty and nonobviousness of the present invention resides in part in the application of DTS (distributed temperature sensing) principles in combination with means for creating conditions within a volume of soil wherein discernment of differences in thermal conductance across a target area is made optimally possible.
Distributed temperature measurement in the invention is accomplished using a system such as can be purchased by York Sensors Limited, York House, Premier Way, Abbey Park, Hampshire SO51 9AQ, UK. With such a system, temperature resolution of about 1C and a spatial resolution of about ½ meter can be obtained, which can be suitable for purposes of the monitoring objectives identified at the outset of this disclosure. Fundamentally, the temperature sensing component works by sending pulses of radiation down a fiber optic (at 1064 nm, in the case of the York system), in OTDR-like fashion, generating Raman scattered radiation throughout. The latter consists of two components, one with a wavelength slightly above 1064 nm and the other slightly below 1064 nm.
It is noted the 1064 nm reference is simply an example. York uses it, but use of that specific wavelength is not a necessity. Other wavelengths could also be used. The wavelength shift involved is well understood by those skilled in the art of OTDR, however, in a general sense, the York sensor utilizes both the "Stokes" component of the scattered radiation (having a wavelength slightly longer than 1064 nm) and the "antiStokes" component (having a wavelength slightly shorter than 1064 nm). These components are shifted by about ±50 nm, respectively, from the 1064 nm. This is the shift that can be observed when using communications grade optical fiber; other materials exhibit different shifts.
The intensity of the two components of the radiation varies with temperature. Hence, the ratio of the anti-Stokes component to the Stokes component depends on temperature. For the optical fiber mentioned, the ratio of anti-Stokes to Stokes is K exp (-700/T), at the point of generation, where T is the absolute temperature and K is a coefficient that depends on the numerical aperture of the fiber and wavelengths raised to the fourth power.
Of course, differences in transmission loss between the two components from the point of generation to the detector have to be taken into account for an accurate temperature measurement. This may be done by means of a "double ended" configuration, by which means scattered radiation moving both forward and backward through the fiber optic is detected at each scattered wavelength. In addition, the time between detected signals and the launching of the initial pulse must be known in order to determine where calculational necessities are built into the system (such as the York system). What is then automatically displayed using the commercial system is a temperature profile along the fiber.
In the present invention, a distributed temperature sensing apparatus such as that just described is integrated with a cable, conduit or other similar linear element that can be positioned according to a serpentine arrangement in a target area within a volume of soil. (It is noted that, for purposes of this disclosure, the phrase "target area" includes not only a two-dimensional geometric area but also a three-dimensional volume.) The linear element may be flexible or rigid, depending on a particular application or user's needs.
One embodiment of the described linear element can include a cable that houses both the optical fiber as well as a heater wire. Other configurations, though, are possible and considered within the scope of the invention and appended claims. Such other configurations include, but are not limited to, arrangements such as those employing a conduit in place of a conventional cable, and configurations wherein the heater wire is omitted, and another form of heater is used in its place. Alternative embodiments also include, for example, deploying a stainless steel tube housing the optical fiber as the heating element. This is described in slightly more detail, below. Also, as mentioned earlier, the heater need not necessarily even be integrated with the linear element, so long as the desired separation in temperature between moist and dry soil can be attained.
The following describes various tests that have been performed which demonstrate the basic operating principles of the invention as well as the effectiveness of several different embodiments.
Each
The remaining discussion that follows describes an intermediate-scale test of the inventive moisture intrusion sensor system that has been conducted to examine its performance under more realistic circumstances than the small-scale tests already discussed. The intermediate-scale test also verifies the use of the conduit as a heater, as well as a protective enclosure for the fiber.
Voltage was then applied in stages to determine the relationship between an anticipated steady-state temperature and electrical power. For a simple system, whose components are unchanging, one expects a linear relationship between the two. The voltages applied were: 25, 50, 75, 100, 115, 130, 151, and 240 volts. The input power is, of course, proportional to the square of these values.
As a final point, the steady-state temperature discussed here can only be a pseudo-steady state because the heating will eventually cause a reduction in moisture content. Thus a true steady state is only achievable after all the moisture has disappeared. However, such a state would yield no information of any value. The pseudo-steady state is both useful and long-lived.
The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the appended claims. It is intended that the scope of the invention be defined by the claims appended hereto. The entire disclosures of all references, applications, patents and publications cited above are hereby incorporated by reference.
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